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IETF RFC 3746
Forwarding and Control Element Separation (ForCES) Framework
Last modified on Monday, April 26th, 2004
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Network Working Group L. Yang
Request for Comments: 3746 Intel Corp.
Category: Informational R. Dantu
Univ. of North Texas
T. Anderson
Intel Corp.
R. Gopal
Nokia
April 2004
Forwarding and Control Element Separation (ForCES) Framework
 Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright © The Internet Society (2004). All Rights Reserved.
Abstract
This document defines the architectural framework for the ForCES
(Forwarding and Control Element Separation) network elements, and
identifies the associated entities and their interactions.
Table of Contents
1. Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions used in this document . . . . . . . . . . . . 2
1.2. Terminologies . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction to Forwarding and Control Element Separation
(ForCES) . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Control Elements and Fr Reference Point . . . . . . . . . 10
3.2. Forwarding Elements and Fi reference point. . . . . . . . 11
3.3. CE Managers . . . . . . . . . . . . . . . . . . . . . . . 14
3.4. FE Managers . . . . . . . . . . . . . . . . . . . . . . . 14
4. Operational Phases . . . . . . . . . . . . . . . . . . . . . . 15
4.1. Pre-association Phase . . . . . . . . . . . . . . . . . . 15
4.1.1. Fl Reference Point . . . . . . . . . . . . . . . . 15
4.1.2. Ff Reference Point . . . . . . . . . . . . . . . . 16
4.1.3. Fc Reference Point . . . . . . . . . . . . . . . . 17
4.2. Post-association Phase and Fp reference point . . . . . . 17
4.2.1. Proximity and Interconnect between CEs and FEs . . 18
Yang, et al. Informational PAGE 1 
RFC 3746 ForCES Framework April 2004
4.2.2. Association Establishment. . . . . . . . . . . . . 18
4.2.3. Steady-state Communication . . . . . . . . . . . . 19
4.2.4. Data Packets across Fp reference point . . . . . . 21
4.2.5. Proxy FE . . . . . . . . . . . . . . . . . . . . . 22
4.3. Association Re-establishment. . . . . . . . . . . . . . . 22
4.3.1. CE graceful restart. . . . . . . . . . . . . . . . 23
4.3.2. FE restart . . . . . . . . . . . . . . . . . . . . 24
5. Applicability to RFC 1812. . . . . . . . . . . . . . . . . . . 25
5.1. General Router Requirements . . . . . . . . . . . . . . . 25
5.2. Link Layer. . . . . . . . . . . . . . . . . . . . . . . . 26
5.3. Internet Layer Protocols. . . . . . . . . . . . . . . . . 27
5.4. Internet Layer Forwarding . . . . . . . . . . . . . . . . 27
5.5. Transport Layer . . . . . . . . . . . . . . . . . . . . . 28
5.6. Application Layer -- Routing Protocols. . . . . . . . . . 29
5.7. Application Layer -- Network Management Protocol. . . . . 29
6. Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30
8. Security Considerations. . . . . . . . . . . . . . . . . . . . 30
8.1. Analysis of Potential Threats Introduced by ForCES. . . . 31
8.1.1. "Join" or "Remove" Message Flooding on CEs . . . . 31
8.1.2. Impersonation Attack . . . . . . . . . . . . . . . 31
8.1.3. Replay Attack. . . . . . . . . . . . . . . . . . . 31
8.1.4. Attack during Fail Over. . . . . . . . . . . . . . 32
8.1.5. Data Integrity . . . . . . . . . . . . . . . . . . 32
8.1.6. Data Confidentiality . . . . . . . . . . . . . . . 32
8.1.7. Sharing security parameters. . . . . . . . . . . . 33
8.1.8. Denial of Service Attack via External Interface. . 33
8.2. Security Recommendations for ForCES . . . . . . . . . . . 33
8.2.1. Using TLS with ForCES. . . . . . . . . . . . . . . 34
8.2.2. Using IPsec with ForCES. . . . . . . . . . . . . . 35
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
9.1. Normative References. . . . . . . . . . . . . . . . . . . 37
9.2. Informative References. . . . . . . . . . . . . . . . . . 37
10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 39
11. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 40
1. Definitions
1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 [1].
Yang, et al. Informational PAGE 2
RFC 3746 ForCES Framework April 2004
1.2. Terminologies
A set of terminology associated with the ForCES requirements is
defined in [4] and we only include the definitions that are most
relevant to this document here.
Addressable Entity (AE) - An entity that is directly addressable
given some interconnect technology. For example, on IP networks, it
is a device to which we can communicate using an IP address; on a
switch fabric, it is a device to which we can communicate using a
switch fabric port number.
Physical Forwarding Element (PFE) - An AE that includes hardware used
to provide per-packet processing and handling. This hardware may
consist of (but is not limited to) network processors, ASICs
(Application-Specific Integrated Circuits), or general purpose
processors, installed on line cards, daughter boards, mezzanine
cards, or in stand-alone boxes.
PFE Partition - A logical partition of a PFE consisting of some
subset of each of the resources (e.g., ports, memory, forwarding
table entries) available on the PFE. This concept is analogous to
that of the resources assigned to a virtual switching element as
described in [9].
Physical Control Element (PCE) - An AE that includes hardware used to
provide control functionality. This hardware typically includes a
general purpose processor.
PCE Partition - A logical partition of a PCE consisting of some
subset of each of the resources available on the PCE.
Forwarding Element (FE) - A logical entity that implements the ForCES
Protocol. FEs use the underlying hardware to provide per-packet
processing and handling as directed by a CE via the ForCES Protocol.
FEs may happen to be a single blade (or PFE), a partition of a PFE,
or multiple PFEs.
Control Element (CE) - A logical entity that implements the ForCES
Protocol and uses it to instruct one or more FEs on how to process
packets. CEs handle functionality such as the execution of control
and signaling protocols. CEs may consist of PCE partitions or whole
PCEs.
ForCES Network Element (NE) - An entity composed of one or more CEs
and one or more FEs. An NE usually hides its internal organization
from external entities and represents a single point of management to
entities outside the NE.
Yang, et al. Informational PAGE 3
RFC 3746 ForCES Framework April 2004
Pre-association Phase - The period of time during which an FE Manager
(see below) and a CE Manager (see below) are determining whether an
FE and a CE should be part of the same network element. It is
possible for some elements of the NE to be in pre-association phase
while other elements are in the post-association phase.
Post-association Phase - The period of time during which an FE knows
which CE is to control it and vice versa, including the time during
which the CE and FE are establishing communication with one another.
ForCES Protocol - While there may be multiple protocols used within
the overall ForCES architecture, the term "ForCES Protocol" refers
only to the ForCES post-association phase protocol (see below).
ForCES Post-Association Phase Protocol - The protocol used for post-
association phase communication between CEs and FEs. This protocol
does not apply to CE-to-CE communication, FE-to-FE communication, or
to communication between FE and CE managers. The ForCES Protocol is
a master-slave protocol in which FEs are slaves and CEs are masters.
This protocol includes both the management of the communication
channel (e.g., connection establishment, heartbeats) and the control
messages themselves. This protocol could be a single protocol or
could consist of multiple protocols working together, and may be
unicast or multicast based. A separate protocol document will
specify this information.
FE Manager - A logical entity that operates in the pre-association
phase and is responsible for determining to which CE(s) an FE should
communicate. This process is called CE discovery and may involve the
FE manager learning the capabilities of available CEs. An FE manager
may use anything from a static configuration to a pre-association
phase protocol (see below) to determine which CE(s) to use; however,
this is currently out of scope. Being a logical entity, an FE
manager might be physically combined with any of the other logical
entities mentioned in this section.
CE Manager - A logical entity that operates in the pre-association
phase and is responsible for determining to which FE(s) a CE should
communicate. This process is called FE discovery and may involve the
CE manager learning the capabilities of available FEs. A CE manager
may use anything from a static configuration to a pre-association
phase protocol (see below) to determine which FE to use; however,
this is currently out of scope. Being a logical entity, a CE manager
might be physically combined with any of the other logical entities
mentioned in this section.
Yang, et al. Informational PAGE 4
RFC 3746 ForCES Framework April 2004
Pre-association Phase Protocol - A protocol between FE managers and
CE managers that is used to determine which CEs or FEs to use. A
pre-association phase protocol may include a CE and/or FE capability
discovery mechanism. Note that this capability discovery process is
wholly separate from (and does not replace) that used within the
ForCES Protocol. However, the two capability discovery mechanisms
may utilize the same FE model.
FE Model - A model that describes the logical processing functions of
an FE.
ForCES Protocol Element - An FE or CE.
Intra-FE topology - Representation of how a single FE is realized by
combining possibly multiple logical functional blocks along multiple
data paths. This is defined by the FE model.
FE Topology - Representation of how the multiple FEs in a single NE
are interconnected. Sometimes it is called inter-FE topology, to be
distinguished from intra-FE topology used by the FE model.
Inter-FE topology - See FE Topology.
2. Introduction to Forwarding and Control Element Separation (ForCES)
An IP network element (NE) appears to external entities as a
monolithic piece of network equipment, e.g., a router, NAT, firewall,
or load balancer. Internally, however, an IP network element (NE)
(such as a router) is composed of numerous logically separated
entities that cooperate to provide a given functionality (such as
routing). Two types of network element components exist: control
element (CE) in control plane and forwarding element (FE) in
forwarding plane (or data plane). Forwarding elements are typically
ASIC, network-processor, or general-purpose processor-based devices
that handle data path operations for each packet. Control elements
are typically based on general-purpose processors that provide
control functionality, like routing and signaling protocols.
ForCES aims to define a framework and associated protocol(s) to
standardize information exchange between the control and forwarding
plane. Having standard mechanisms allows CEs and FEs to become
physically separated standard components. This physical separation
accrues several benefits to the ForCES architecture. Separate
components would allow component vendors to specialize in one
component without having to become experts in all components.
Standard protocol also allows the CEs and FEs from different
component vendors to interoperate with each other and hence it
becomes possible for system vendors to integrate together the CEs and
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RFC 3746 ForCES Framework April 2004
FEs from different component suppliers. This interoperability
translates into increased design choices and flexibility for the
system vendors. Overall, ForCES will enable rapid innovation in both
the control and forwarding planes while maintaining interoperability.
Scalability is also easily provided by this architecture in that
additional forwarding or control capacity can be added to existing
network elements without the need for forklift upgrades.
------------------------- -------------------------
| Control Blade A | | Control Blade B |
| (CE) | | (CE) |
------------------------- -------------------------
^ | ^ |
| | | |
| V | V
---------------------------------------------------------
| Switch Fabric Backplane |
---------------------------------------------------------
^ | ^ | ^ |
| | | | . . . | |
| V | V | V
------------ ------------ ------------
|Router | |Router | |Router |
|Blade #1 | |Blade #2 | |Blade #N |
| (FE) | | (FE) | | (FE) |
------------ ------------ ------------
^ | ^ | ^ |
| | | | . . . | |
| V | V | V
Figure 1. A router configuration example with separate blades.
One example of such physical separation is at the blade level. Figure
1 shows such an example configuration of a router, with two control
blades and multiple forwarding blades, all interconnected into a
switch fabric backplane. In such a chassis configuration, the
control blades are the CEs while the router blades are the FEs, and
the switch fabric backplane provides the physical interconnect for
all the blades. Control blade A may be the primary CE while control
blade B may be the backup CE providing redundancy. It is also
possible to have a redundant switch fabric for high availability
support. Routers today with this kind of configuration use
proprietary interfaces for messaging between CEs and FEs. The goal
of ForCES is to replace such proprietary interfaces with a standard
protocol. With a standard protocol like ForCES implemented on all
blades, it becomes possible for control blades from vendor X and
forwarding blades from vendor Y to work seamlessly together in one
chassis.
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RFC 3746 ForCES Framework April 2004
------- -------
| CE1 | | CE2 |
------- -------
^ ^
| |
V V
============================================ Ethernet
^ ^ . . . ^
| | |
V V V
------- ------- --------
| FE#1| | FE#2| | FE#n |
------- ------- --------
^ | ^ | ^ |
| | | | | |
| V | V | V
Figure 2. A router configuration example with separate boxes.
Another level of physical separation between the CEs and FEs can be
at the box level. In such a configuration, all the CEs and FEs are
physically separated boxes, interconnected with some kind of high
speed LAN connection (like Gigabit Ethernet). These separated CEs
and FEs are only one hop away from each other within a local area
network. The CEs and FEs communicate to each other by running
ForCES, and the collection of these CEs and FEs together become one
routing unit to the external world. Figure 2 shows such an example.
In both examples shown here, the same physical interconnect is used
for both CE-to-FE and FE-to-FE communication. However, that does not
have to be the case. One reason to use different interconnects is
that the CE-to-FE interconnect does not have to be as fast as the
FE-to-FE interconnect, so the more faster and more expensive
connections can be saved for FE-to-FE. The separate interconnects
may also provide reliability and redundancy benefits for the NE.
Some examples of control functions that can be implemented in the CE
include routing protocols like RIP, OSPF, and BGP, control and
signaling protocols like RSVP (Resource Reservation Protocol), LDP
(Label Distribution Protocol) for MPLS, etc. Examples of forwarding
functions in the FE include LPM (longest prefix match) forwarder,
classifiers, traffic shaper, meter, NAT (Network Address
Translators), etc. Figure 3 provides example functions in both CE
and FE. Any given NE may contain one or many of these CE and FE
functions in it. The diagram also shows that the ForCES Protocol is
used to transport both the control messages for ForCES itself and the
Yang, et al. Informational PAGE 7
RFC 3746 ForCES Framework April 2004
data packets that are originated/destined from/to the control
functions in the CE (e.g., routing packets). Section 4.2.4 provides
more detail on this.
-------------------------------------------------
| | | | | | |
|OSPF |RIP |BGP |RSVP |LDP |. . . |
| | | | | | |
-------------------------------------------------
| ForCES Interface |
-------------------------------------------------
^ ^
ForCES | |data
control | |packets
messages| |(e.g., routing packets)
v v
-------------------------------------------------
| ForCES Interface |
-------------------------------------------------
| | | | | | |
|LPM Fwd|Meter |Shaper |NAT |Classi-|. . . |
| | | | |fier | |
-------------------------------------------------
| FE resources |
-------------------------------------------------
Figure 3. Examples of CE and FE functions.
A set of requirements for control and forwarding separation is
identified in [4]. This document describes a ForCES architecture
that satisfies the architectural requirements of [4] and defines a
framework for ForCES network elements and the associated entities to
facilitate protocol definition. Whenever necessary, this document
uses many examples to illustrate the issues and/or possible solutions
in ForCES. These examples are intended to be just examples, and
should not be taken as the only or definite ways of doing certain
things. It is expected that a separate document will be produced by
the ForCES working group to specify the ForCES Protocol.
3. Architecture
This section defines the ForCES architectural framework and the
associated logical components. This ForCES framework defines
components of ForCES NEs, including several ancillary components.
These components may be connected in different kinds of topologies
for flexible packet processing.
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RFC 3746 ForCES Framework April 2004
---------------------------------------
| ForCES Network Element |
-------------- Fc | -------------- -------------- |
| CE Manager |---------+-| CE 1 |------| CE 2 | |
-------------- | | | Fr | | |
| | -------------- -------------- |
| Fl | | | Fp / |
| | Fp| |----------| / |
| | | |/ |
| | | | |
| | | Fp /|----| |
| | | /--------/ | |
-------------- Ff | -------------- -------------- |
| FE Manager |---------+-| FE 1 | Fi | FE 2 | |
-------------- | | |------| | |
| -------------- -------------- |
| | | | | | | | | |
----+--+--+--+----------+--+--+--+-----
| | | | | | | |
| | | | | | | |
Fi/f Fi/f
Fp: CE-FE interface
Fi: FE-FE interface
Fr: CE-CE interface
Fc: Interface between the CE Manager and a CE
Ff: Interface between the FE Manager and an FE
Fl: Interface between the CE Manager and the FE Manager
Fi/f: FE external interface
Figure 4. ForCES Architectural Diagram
The diagram in Figure 4 shows the logical components of the ForCES
architecture and their relationships. There are two kinds of
components inside a ForCES network element: control element (CE) and
forwarding element (FE). The framework allows multiple instances of
CE and FE inside one NE. Each FE contains one or more physical media
interfaces for receiving and transmitting packets from/to the
external world. The aggregation of these FE interfaces becomes the
NE's external interfaces. In addition to the external interfaces,
there must also exist some kind of interconnect within the NE so that
the CE and FE can communicate with each other, and one FE can forward
packets to another FE. The diagram also shows two entities outside
of the ForCES NE: CE Manager and FE Manager. These two ancillary
entities provide configuration to the corresponding CE or FE in the
pre-association phase (see Section 4.1).
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RFC 3746 ForCES Framework April 2004
For convenience, the logical interactions between these components
are labeled by reference points Fp, Fc, Ff, Fr, Fl, and Fi, as shown
in Figure 4. The FE external interfaces are labeled as Fi/f. More
detail is provided in Section 3 and 4 for each of these reference
points. All these reference points are important in understanding
the ForCES architecture, however, the ForCES Protocol is only defined
over one reference point -- Fp.
The interface between two ForCES NEs is identical to the interface
between two conventional routers and these two NEs exchange the
protocol packets through the external interfaces at Fi/f. ForCES NEs
connect to existing routers transparently.
3.1. Control Elements and Fr Reference Point
It is not necessary to define any protocols across the Fr reference
point to enable control and forwarding separation for simple
configurations like single CE and multiple FEs. However, this
architecture permits multiple CEs to be present in a network element.
In cases where an implementation uses multiple CEs, the invariant
that the CEs and FEs together appear as a single NE must be
maintained.
Multiple CEs may be used for redundancy, load sharing, distributed
control, or other purposes. Redundancy is the case where one or more
CEs are prepared to take over should an active CE fail. Load sharing
is the case where two or more CEs are concurrently active and any
request that can be serviced by one of the CEs can also be serviced
by any of the other CEs. For both redundancy and load sharing, the
CEs involved are equivalently capable. The only difference between
these two cases is in terms of how many active CEs there are
simultaneously. Distributed control is the case where two or more
CEs are concurrently active but certain requests can only be serviced
by certain CEs.
When multiple CEs are employed in a ForCES NE, their internal
organization is considered an implementation issue that is beyond the
scope of ForCES. CEs are wholly responsible for coordinating amongst
themselves via the Fr reference point to provide consistency and
synchronization. However, ForCES does not define the implementation
or protocols used between CEs, nor does it define how to distribute
functionality among CEs. Nevertheless, ForCES will support
mechanisms for CE redundancy or fail over, and it is expected that
vendors will provide redundancy or fail over solutions within this
framework.
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RFC 3746 ForCES Framework April 2004
3.2. Forwarding Elements and Fi reference point
An FE is a logical entity that implements the ForCES Protocol and
uses the underlying hardware to provide per-packet processing and
handling as directed by a CE. It is possible to partition one
physical FE into multiple logical FEs. It is also possible for one
FE to use multiple physical FEs. The mapping between physical FE(s)
and logical FE(s) is beyond the scope of ForCES. For example, a
logical partition of a physical FE can be created by assigning some
portion of each of the resources (e.g., ports, memory, forwarding
table entries) available on the ForCES physical FE to each of the
logical FEs. Such a concept of FE virtualization is analogous to a
virtual switching element as described in [9]. If FE virtualization
occurs only in the pre-association phase, it has no impact on ForCES.
However, if FE virtualization results in a resource change taken from
an existing FE (already participating in ForCES post-association
phase), the ForCES Protocol needs to be able to inform the CE of such
a change via asynchronous messages (see [4], Section 5, requirement
#6).
FEs perform all packet processing functions as directed by CEs. FEs
have no initiative of their own. Instead, FEs are slaves and only do
as they are told. FEs may communicate with one or more CEs
concurrently across reference point Fp. FEs have no notion of CE
redundancy, load sharing, or distributed control. Instead, FEs
accept commands from any CE authorized to control them, and it is up
to the CEs to coordinate among themselves to achieve redundancy, load
sharing, or distributed control. The idea is to keep FEs as simple
and dumb as possible so that FEs can focus their resources on the
packet processing functions. Unless otherwise configured or
determined by a ForCEs Protocol exchange, each FE will process
authorized incoming commands directed at it as it receives them on a
first come first serve basis.
For example, in Figure 5, FE1 and FE2 can be configured to accept
commands from both the primary CE (CE1) and the backup CE (CE2).
Upon detection of CE1 failure, perhaps across the Fr or Fp reference
point, CE2 is configured to take over activities of CE1. This is
beyond the scope of ForCES and is not discussed further.
Distributed control can be achieved in a similar fashion, without
much intelligence on the part of FEs. For example, FEs can be
configured to detect RSVP and BGP protocol packets, and forward RSVP
packets to one CE and BGP packets to another CE. Hence, FEs may need
to do packet filtering for forwarding packets to specific CEs.
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RFC 3746 ForCES Framework April 2004
------- Fr -------
| CE1 | ------| CE2 |
------- -------
| \ / |
| \ / |
| \ / |
| \/Fp |
| /\ |
| / \ |
| / \ |
------- Fi -------
| FE1 |<----->| FE2 |
------- -------
Figure 5. CE redundancy example.
This architecture permits multiple FEs to be present in an NE. [4]
dictates that the ForCES Protocol must be able to scale to at least
hundreds of FEs (see [4] Section 5, requirement #11). Each of these
FEs may potentially have a different set of packet processing
functions, with different media interfaces. FEs are responsible for
basic maintenance of layer-2 connectivity with other FEs and with
external entities. Many layer-2 media include sophisticated control
protocols. The FORCES Protocol (over the Fp reference point) will be
able to carry messages for such protocols so that, in keeping with
the dumb FE model, the CE can provide appropriate intelligence and
control over these media.
When multiple FEs are present, ForCES requires that packets must be
able to arrive at the NE by one FE and leave the NE via a different
FE (See [4], Section 5, Requirement #3). Packets that enter the NE
via one FE and leave the NE via a different FE are transferred
between FEs across the Fi reference point. The Fi reference point
could be used by FEs to discover their (inter-FE) topology, perhaps
during the pre-association phase. The Fi reference point is a
separate protocol from the Fp reference point and is not currently
defined by the ForCES Protocol.
FEs could be connected in different kinds of topologies and packet
processing may spread across several FEs in the topology. Hence,
logical packet flow may be different from physical FE topology.
Figure 6 provides some topology examples. When it is necessary to
forward packets between FEs, the CE needs to understand the FE
topology. The FE topology may be queried from the FEs by the CEs via
the ForCES Protocol, but the FEs are not required to provide that
information to the CEs. So, the FE topology information may also be
gathered by other means outside of the ForCES Protocol (like inter-FE
topology discovery protocol).
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RFC 3746 ForCES Framework April 2004
-----------------
| CE |
-----------------
^ ^ ^
/ | \
/ v \
/ ------- \
/ +->| FE3 |<-+ \
/ | | | | \
v | ------- | v
------- | | -------
| FE1 |<-+ +->| FE2 |
| |<--------------->| |
------- -------
^ | ^ |
| | | |
| v | v
(a) Full mesh among FE1, FE2, and FE3
-----------
| CE |
-----------
^ ^ ^ ^
/ | | \
/------ | | ------\
v v v v
------- ------- ------- -------
| FE1 |<->| FE2 |<->| FE3 |<->| FE4 |
------- ------- ------- -------
^ | ^ | ^ | ^ |
| | | | | | | |
| v | v | v | v
(b) Multiple FEs in a daisy chain
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RFC 3746 ForCES Framework April 2004
^ |
| v
-----------
| FE1 |<-----------------------|
----------- |
^ ^ |
/ \ |
| ^ / \ ^ | V
v | v v | v ----------
--------- --------- | |
| FE2 | | FE3 |<------------>| CE |
--------- --------- | |
^ ^ ^ ----------
| \ / ^ ^
| \ / | |
| v v | |
| ----------- | |
| | FE4 |<----------------------| |
| ----------- |
| | ^ |
| v | |
| |
|----------------------------------------|
(c) Multiple FEs connected by a ring
Figure 6. Some examples of FE topology
3.3. CE Managers
CE managers are responsible for determining which FEs a CE should
control. It is legitimate for CE managers to be hard-coded with the
knowledge of with which FEs its CEs should communicate with. A CE
manager may also be physically embedded into a CE and be implemented
as a simple keypad or other direct configuration mechanism on the CE.
Finally, CE managers may be physically and logically separate
entities that configure the CE with FE information via such
mechanisms as COPS-PR [7] or SNMP [5].
3.4. FE Managers
FE managers are responsible for determining with which CE any
particular FE should initially communicate. Like CE managers, no
restrictions are placed on how an FE manager decides with which CE
its FEs should communicate, nor are restrictions placed on how FE
managers are implemented. Each FE should have one and only one FE
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RFC 3746 ForCES Framework April 2004
manager, while different FEs may have the same or different FE
manager(s). Each manager can choose to exist and operate
independently of other manager.
4. Operational Phases
Both FEs and CEs require some configuration to be in place before
they can start information exchange and function as a coherent
network element. Two operational phases are identified in this
framework: pre-association and post-association.
4.1. Pre-association Phase
The Pre-association phase is the period of time during which an FE
Manager and a CE Manager are determining whether an FE and a CE
should be part of the same network element. The protocols used
during this phase may include all or some of the message exchange
over Fl, Ff, and Fc reference points. However, all these may be
optional and none of this is within the scope of the ForCES Protocol.
4.1.1. Fl Reference Point
CE managers and FE managers may communicate across the Fl reference
point in the pre-association phase in order to determine whether an
individual CE and FE, or a set of CEs and FEs should be associated.
Communication across the Fl reference point is optional in this
architecture. No requirements are placed on this reference point.
CE managers and FE managers may be operated by different entities.
The operator of the CE manager may not want to divulge, except to
specified FE managers, any characteristics of the CEs it manages.
Similarly, the operator of the FE manager may not want to divulge FE
characteristics, except to authorized entities. As such, CE managers
and FE managers may need to authenticate one another. Subsequent
communication between CE managers and FE managers may require other
security functions such as privacy, non-repudiation, freshness, and
integrity.
Yang, et al. Informational PAGE 15
RFC 3746 ForCES Framework April 2004
FE Manager FE CE Manager CE
| | | |
| | | |
|(security exchange) | |
1|<------------------------------>| |
| | | |
|(a list of CEs and their attributes) |
2|<-------------------------------| |
| | | |
|(a list of FEs and their attributes) |
3|------------------------------->| |
| | | |
| | | |
|<----------------Fl------------>| |
Figure 7. An example of a message exchange over the Fl reference
point
Once the necessary security functions have been performed, the CE and
FE managers communicate to determine which CEs and FEs should
communicate with each other. At the very minimum, the CE and FE
managers need to learn of the existence of available FEs and CEs
respectively. This discovery process may entail one or both managers
learning the capabilities of the discovered ForCES protocol elements.
Figure 7 shows an example of a possible message exchange between the
CE manager and FE manager over the Fl reference point.
4.1.2. Ff Reference Point
The Ff reference point is used to inform forwarding elements of the
association decisions made by the FE manager in the pre-association
phase. Only authorized entities may instruct an FE with respect to
which CE should control it. Therefore, privacy, integrity,
freshness, and authentication are necessary between the FE manager
and FEs when the FE manager is remote to the FE. Once the
appropriate security has been established, the FE manager instructs
the FEs across this reference point to join a new NE or to disconnect
from an existing NE. The FE Manager could also assign unique FE
identifiers to the FEs using this reference point. The FE
identifiers are useful in the post association phase to express FE
topology. Figure 8 shows example of a message exchange over the Ff
reference point.
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RFC 3746 ForCES Framework April 2004
FE Manager FE CE Manager CE
| | | |
| | | |
|(security exchange) |(security exchange)
1|<------------>|authentication 1|<----------->|authentication
| | | |
|(FE ID, attributes) |(CE ID, attributes)
2|<-------------|request 2|<------------|request
| | | |
3|------------->|response 3|------------>|response
|(corresponding CE ID) |(corresponding FE ID)
| | | |
| | | |
|<-----Ff----->| |<-----Fc---->|
Figure 8. Examples of a message exchange
over the Ff and Fc reference points
Note that the FE manager function may be co-located with the FE (such
as by manual keypad entry of the CE IP address), in which case this
reference point is reduced to a built-in function.
4.1.3. Fc Reference Point
The Fc reference point is used to inform control elements of the
association decisions made by CE managers in the pre-association
phase. When the CE manager is remote, only authorized entities may
instruct a CE to control certain FEs. Privacy, integrity, freshness,
and authentication are also required across this reference point in
such a configuration. Once appropriate security has been
established, the CE manager instructs the CEs as to which FEs they
should control and how they should control them. Figure 8 shows
example of a message exchange over the Fc reference point.
As with the FE manager and FEs, configurations are possible where the
CE manager and CE are co-located and no protocol is used for this
function.
4.2. Post-association Phase and Fp reference point
The Post-association phase is the period of time during which an FE
and CE have been configured with information necessary to contact
each other and includes both association establishment and steady-
state communication. The communication between CE and FE is
performed across the Fp ("p" meaning protocol) reference point.
ForCES Protocol is exclusively used for all communication across the
Fp reference point.
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4.2.1. Proximity and Interconnect between CEs and FEs
The ForCES Working Group has made a conscious decision that the first
version of ForCES will be focused on "very close" CE/FE localities in
IP networks. Very Close localities consist of control and forwarding
elements that are either components in the same physical box, or
separated at most by one local network hop ([8]). CEs and FEs can be
connected by a variety of interconnect technologies, including
Ethernet connections, backplanes, ATM (cell) fabrics, etc. ForCES
should be able to support each of these interconnects (see [4]
Section 5, requirement #1). When the CEs and FEs are separated
beyond a single L3 routing hop, the ForCES Protocol will make use of
an existing RFC 2914 [3] compliant L4 protocol with adequate
reliability, security, and congestion control (e.g., TCP, SCTP) for
transport purposes.
4.2.2. Association Establishment
FE CE
| |
|(Security exchange.) |
1|<--------------------->|
| |
|(Let me join the NE please.)
2|---------------------->|
| |
|(What kind of FE are you? -- capability query)
3|<----------------------|
| |
|(Here is my FE functions/state: use model to
describe)
4|---------------------->|
| |
|(Initial config for FE -- optional)
5|<----------------------|
| |
|(I am ready to go. Shall I?)
6|---------------------->|
| |
|(Go ahead!) |
7|<----------------------|
| |
Figure 9. Example of a message exchange between CE and FE
over Fp to establish an NE association
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As an example, figure 9 shows some of the message exchange that may
happen before the association between the CE and FE is fully
established. Either the CE or FE can initiate the connection.
Security handshake is necessary to authenticate the two communication
endpoints to each other before any further message exchange can
happen. The security handshake should include mutual authentication
and authorization between the CE and FE, but the exact details depend
on the security solution chosen by the ForCES Protocol.
Authorization can be as simple as checking against the list of
authorized end points provided by the FE or CE manager during the
pre-association phase. Both authentication and authorization must be
successful before the association can be established. If either
authentication or authorization fails, the end point must not be
allowed to join the NE. After the successful security handshake,
message authentication and confidentiality are still necessary for
the on-going information exchange between the CE and FE, unless some
form of physical security exists. Whenever a packet fails
authentication, it must be dropped and a notification may be sent to
alert the sender of the potential attack. Section 8 provides more
details on the security considerations for ForCES.
After the successful security handshake, the FE needs to inform the
CE of its own capability and optionally its topology in relation to
other FEs. The capability of the FE shall be represented by the FE
model, as required in [4] (Section 6, requirement #1). The model
would allow an FE to describe what kind of packet processing
functions it contains, in what order the processing happens, what
kinds of configurable parameters it allows, what statistics it
collects, and what events it might throw, etc. Once such information
is available to the CE, the CE may choose to send some initial or
default configuration to the FE so that the FE can start receiving
and processing packets correctly. Such initialization may not be
necessary if the FE already obtains the information from its own
bootstrap process. Once the necessary initial information is
exchanged, the process of association is completed. Packet
processing and forwarding at the FE cannot begin until association is
established. After the association is established, the CE and FE
enter steady-state communication.
4.2.3. Steady-state Communication
Once an association is established between the CE and FE, the ForCES
Protocol is used by the CE and FE over the Fp reference point to
exchange information to facilitate packet processing.
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FE CE
| |
|(Add these new routes.)|
1|<----------------------|
| |
|(Successful.) |
2|---------------------->|
| |
| |
|(Query some stats.) |
1|<----------------------|
| |
|(Reply with stats collected.)
2|---------------------->|
| |
| |
|(My port is down, with port #.)
1|---------------------->|
| |
|(Here is a new forwarding table)
2|<----------------------|
| |
Figure 10. Examples of a message exchange between CE and FE
over Fp during steady-state communication
Based on the information acquired through CEs' control processing,
CEs will frequently need to manipulate the packet-forwarding
behaviors of their FE(s) by sending instructions to FEs. For
example, Figure 10 shows message exchange examples in which the CE
sends new routes to the FE so that the FE can add them to its
forwarding table. The CE may query the FE for statistics collected
by the FE and the FE may notify the CE of important events such as
port failure.
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4.2.4. Data Packets across Fp reference point
--------------------- ----------------------
| | | |
| +--------+ | | +--------+ |
| |CE(BGP) | | | |CE(BGP) | |
| +--------+ | | +--------+ |
| | | | ^ |
| |Fp | | |Fp |
| v | | | |
| +--------+ | | +--------+ |
| | FE | | | | FE | |
| +--------+ | | +--------+ |
| | | | ^ |
| Router | | | Router | |
| A | | | B | |
---------+----------- -----------+----------
v ^
| |
| |
------------------->---------------
Figure 11. Example to show data packet flow between two NEs.
Control plane protocol packets (such as RIP, OSPF messages) addressed
to any of NE's interfaces are typically redirected by the receiving
FE to its CE, and CE may originate packets and have its FE deliver
them to other NEs. Therefore, the ForCES Protocol over Fp not only
transports the ForCES Protocol messages between CEs and FEs, but also
encapsulates the data packets from control plane protocols.
Moreover, one FE may be controlled by multiple CEs for distributed
control. In this configuration, the control protocols supported by
the FORCES NEs may spread across multiple CEs. For example, one CE
may support routing protocols like OSPF and BGP, while a signaling
and admission control protocol like RSVP is supported in another CE.
FEs are configured to recognize and filter these protocol packets and
forward them to the corresponding CE.
Figure 11 shows one example of how the BGP packets originated by
router A are passed to router B. In this example, the ForCES
Protocol is used to transport the packets from the CE to the FE
inside router A, and then from the FE to the CE inside router B. In
light of the fact that the ForCES Protocol is responsible for
transporting both the control messages and the data packets between
the CE and FE over the Fp reference point, it is possible to use
either a single protocol or multiple protocols.
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4.2.5. Proxy FE
In the case where a physical FE cannot implement (e.g., due to the
lack of a general purpose CPU) the ForCES Protocol directly, a proxy
FE can be used to terminate the Fp reference point instead of the
physical FE. This allows the CE to communicate to the physical FE
via the proxy by using ForCES, while the proxy manipulates the
physical FE using some intermediary form of communication (e.g., a
non-ForCES protocol or DMA). In such an implementation, the
combination of the proxy and the physical FE becomes one logical FE
entity. It is also possible for one proxy to act on behalf of
multiple physical FEs.
One needs to be aware of the security implication introduced by the
proxy FE. Since the physical FE is not capable of implementing
ForCES itself, the security mechanism of ForCES can only secure the
communication channel between the CE and the proxy FE, but not all
the way to the physical FE. It is recommended that other security
mechanisms (including physical security property) be employed to
ensure the security between the CE and the physical FE.
4.3. Association Re-establishment
FEs and CEs may join and leave NEs dynamically (see [4] Section 5,
requirements #12). When an FE or CE leaves the NE, the association
with the NE is broken. If the leaving party rejoins an NE later, to
re-establish the association, it may need to re-enter the pre-
association phase. Loss of association can also happen unexpectedly
due to a loss of connection between the CE and the FE. Therefore,
the framework allows the bi-directional transition between these two
phases, but the ForCES Protocol is only applicable for the post-
association phase. However, the protocol should provide mechanisms
to support association re-establishment. This includes the ability
for CEs and FEs to determine when there is a loss of association
between them, and to restore association and efficient state
(re)synchronization mechanisms (see [4] Section 5, requirement #7).
Note that security association and state must also be re-established
to guarantee the same level of security (including both
authentication and authorization) exists before and after the
association re-establishment.
When an FE leaves or joins an existing NE that is already in
operation, the CE needs to be aware of the impact on FE topology and
deal with the change accordingly.
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4.3.1. CE graceful restart
The failure and restart of the CE in a router can potentially cause
much stress and disruption on the control plane throughout a network
because in restarting a CE for any reason, the router loses routing
adjacencies or sessions with its routing neighbors. Neighbors who
detect the lost adjacency normally re-compute new routes and then
send routing updates to their own neighbors to communicate the lost
adjacency. Their neighbors do the same thing to propagate throughout
the network. In the meantime, the restarting router cannot receive
traffic from other routers because the neighbors have stopped using
the router's previously advertised routes. When the restarting
router restores adjacencies, neighbors must once again re-compute new
routes and send out additional routing updates. The restarting
router is unable to forward packets until it has re-established
routing adjacencies with neighbors, received route updates through
these adjacencies, and computed new routes. Until convergence takes
place throughout the network, packets may be lost in transient black
holes or forwarding loops.
A high availability mechanism known as the "graceful restart" has
been used by the IP routing protocols (OSPF [11], BGP [12], IS-IS
[13]) and MPLS label distribution protocol (LDP [10]) to help
minimize the negative effects on routing throughout an entire network
caused by a restarting router. Route flap on neighboring routers is
avoided, and a restarting router can continue to forward packets that
would otherwise be dropped.
While the details differ from protocol to protocol, the general idea
behind the graceful restart mechanism remains the same. With the
graceful restart, a restarting router can inform its neighbors when
it restarts. The neighbors may detect the lost adjacency but do not
recompute new routes or send routing updates to their neighbors. The
neighbors also hold on to the routes received from the restarting
router before restart and assume they are still valid for a limited
time. By doing so, the restarting router's FEs can also continue to
receive and forward traffic from other neighbors for a limited time
by using the routes they already have. The restarting router then
re-establishes routing adjacencies, downloads updated routes from all
its neighbors, recomputes new routes, and uses them to replace the
older routes it was using. It then sends these updated routes to its
neighbors and signals the completion of the graceful restart process.
Non-stop forwarding is a requirement for graceful restart. It is
necessary so a router can continue to forward packets while it is
downloading routing information and recomputing new routes. This
ensures that packets will not be dropped. As one can see, one of the
benefits afforded by the separation of CE and FE is exactly the
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ability of non-stop forwarding in the face of the CE failure and
restart. The support of dynamic changes to CE/FE association in
ForCES also makes it compatible with high availability mechanisms,
such as graceful restart.
ForCES should be able to support a CE graceful restart easily. When
the association is established the first time, the CE must inform the
FEs what to do in the case of a CE failure. If graceful restart is
not supported, the FEs may be told to stop packet processing all
together if its CE fails. If graceful restart is supported, the FEs
should be told to cache and hold on to its FE state, including the
forwarding tables across the restarts. A timer must be included so
that the timeout causes such a cached state to eventually expire.
Those timers should be settable by the CE.
4.3.2. FE restart
In the same example in Figure 5, assuming CE1 is the working CE for
the moment, what would happen if one of the FEs, say FE1, leaves the
NE temporarily? FE1 may voluntarily decide to leave the association.
Alternatively, FE1 may stop functioning simply due to unexpected
failure. In the former case, CE1 receives a "leave-association
request" from FE1. In the latter, CE1 detects the failure of FE1 by
some other means. In both cases, CE1 must inform the routing
protocols of such an event, most likely prompting a reachability and
SPF (Shortest Path First) recalculation and associated downloading of
new FIBs from CE1 to the other remaining FEs (only FE2 in this
example). Such recalculation and FIB updates will also be propagated
from CE1 to the NE's neighbors that are affected by the connectivity
of FE1.
When FE1 decides to rejoin again, or when it restarts again after the
failure, FE1 needs to re-discover its master (CE). This can be
achieved by several means. It may re-enter the pre-association phase
and get that information from its FE manager. It may retrieve the
previous CE information from its cache, if it can validate the
information freshness. Once it discovers its CE, it starts message
exchange with the CE to re-establish the association, as outlined in
Figure 9, with the possible exception that it might be able to bypass
the transport of the complete initial configuration. Suppose that
FE1 still has its routing table and other state information from the
last association. Instead of re-sending all the information, it may
be able to use a more efficient mechanism to re-sync the state with
its CE, if such a mechanism is supported by the ForCES Protocol. For
example, CRC-32 of the state might give a quick indication of whether
or not the state is in-sync with its CE. By comparing its state with
the CE first, it sends an information update
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RFC 3746 ForCES Framework April 2004
only if it is needed. The ForCES Protocol may choose to implement
similar optimization mechanisms, but it may also choose not to, as
this is not a requirement.
5. Applicability to RFC 1812
[4] Section 5, requirement #9 dictates "Any proposed ForCES
architecture must explain how that architecture supports all of the
router functions as defined in RFC 1812." RFC 1812 [2] discusses
many important requirements for IPv4 routers from the link layer to
the application layer. This section addresses the relevant
requirements in RFC 1812 for implementing IPv4 routers based on
ForCES architecture and explains how ForCES satisfies these
requirements by providing guidelines on how to separate the
functionalities required into the forwarding plane and control plane.
In general, the forwarding plane carries out the bulk of the per-
packet processing that is required at line speed, while the control
plane carries most of the computationally complex operations that are
typical of the control and signaling protocols. However, it is
impossible to draw a rigid line to divide the processing into CEs and
FEs cleanly and the ForCES architecture should not limit the
innovative approaches in control and forwarding plane separation. As
more and more processing power is available in the FEs, some of the
control functions that traditionally are performed by CEs may now be
moved to FEs for better performance and scalability. Such offloaded
functions may include part of ICMP or TCP processing, or part of
routing protocols. Once off-loaded onto the forwarding plane, such
CE functions, even though logically belonging to the control plane,
now become part of the FE functions. Just like the other logical
functions performed by FEs, such off-loaded functions must be
expressed as part of the FE model so that the CEs can decide how to
best take advantage of these off-loaded functions when present on the
FEs.
5.1. General Router Requirements
Routers have at least two or more logical interfaces. When CEs and
FEs are separated by ForCES within a single NE, some additional
interfaces are needed for intra-NE communications, as illustrated in
figure 12. This NE contains one CE and two FEs. Each FE has four
interfaces; two of them are used for receiving and transmitting
packets to the external world, while the other two are for intra-NE
connections. CE has two logical interfaces #9 and #10, connected to
interfaces #3 and #6 from FE1 and FE2, respectively. Interface #4
and #5 are connected for FE1-FE2 communication. Therefore, this
router NE provides four external interfaces (#1, 2, 7, and 8).
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RFC 3746 ForCES Framework April 2004
---------------------------------
| router NE |
| ----------- ----------- |
| | FE1 | | FE2 | |
| ----------- ----------- |
| 1| 2| 3| 4| 5| 6| 7| 8| |
| | | | | | | | | |
| | | | +----+ | | | |
| | | | | | | |
| | | 9| 10| | | |
| | | -------------- | | |
| | | | CE | | | |
| | | -------------- | | |
| | | | | |
-----+--+----------------+--+----
| | | |
| | | |
Figure 12. A router NE example with four interfaces.
IPv4 routers must implement IP to support its packet forwarding
function, which is driven by its FIB (Forwarding Information Base).
This Internet layer forwarding (see RFC 1812 [2] Section 5)
functionality naturally belongs to FEs in the ForCES architecture.
A router may implement transport layer protocols (like TCP and UDP)
that are required to support application layer protocols (see RFC
1812 [2] Section 6). One important class of application protocols is
routing protocols (see RFC 1812 [2] Section 7). In the ForCES
architecture, routing protocols are naturally implemented by CEs.
Routing protocols require that routers communicate with each other.
This communication between CEs in different routers is supported in
ForCES by FEs' ability to redirect data packets addressed to routers
(i.e., NEs), and the CEs' ability to originate packets and have them
delivered by their FEs. This communication occurs across the Fp
reference point inside each router and between neighboring routers'
external interfaces, as illustrated in Figure 11.
5.2. Link Layer
Since FEs own all the external interfaces for the router, FEs need to
conform to the link layer requirements in RFC 1812 [2]. Arguably,
ARP support may be implemented in either CEs or FEs. As we will see
later, a number of behaviors that RFC 1812 mandates fall into this
category -- they may be performed by the FE and may be performed by
the CE. A general guideline is needed to ensure interoperability
between separated control and forwarding planes. The guideline we
offer here is that CEs MUST be capable of these kinds of operations
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RFC 3746 ForCES Framework April 2004
while FEs MAY choose to implement them. The FE model should indicate
its capabilities in this regard so that CEs can decide where these
functions are implemented.
Interface parameters, including MTU, IP address, etc., must be
configurable by CEs via ForCES. CEs must be able to determine
whether a physical interface in an FE is available to send packets or
not. FEs must also inform CEs of the status change of the interfaces
(like link up/down) via ForCES.
5.3. Internet Layer Protocols
Both FEs and CEs must implement the IP protocol and all mandatory
extensions as RFC 1812 specified. CEs should implement IP options
like source route and record route while FEs may choose to implement
those as well. The timestamp option should be implemented by FEs to
insert the timestamp most accurately. The FE must interpret the IP
options that it understands and preserve the rest unchanged for use
by CEs. Both FEs and CEs might choose to silently discard packets
without sending ICMP errors, but such events should be logged and
counted. FEs may report statistics for such events to CEs via
ForCES.
When multiple FEs are involved to process packets, the appearance of
a single NE must be strictly maintained. For example, Time-To-Live
(TTL) must be decremented only once within a single NE. For example,
it can be always decremented by the last FE with egress function.
FEs must receive and process normally any packets with a broadcast
destination address or a multicast destination address that the
router has asked to receive. When IP multicast is supported in
routers, IGMP is implemented in CEs. CEs are also required of ICMP
support, while it is optional for FEs to support ICMP. Such an
option can be communicated to CEs as part of the FE model. Therefore,
FEs can always rely upon CEs to send out ICMP error messages, but FEs
also have the option of generating ICMP error messages themselves.
5.4. Internet Layer Forwarding
IP forwarding is implemented by FEs. When the routing table is
updated at the CEs, ForCES is used to send the new route entries from
the CEs to FEs. Each FE has its own forwarding table and uses this
table to direct packets to the next hop interface.
Upon receiving IP packets, the FE verifies the IP header and
processes most of the IP options. Some options cannot be processed
until the routing decision has been made. The routing decision is
made after examining the destination IP address. If the destination
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address belongs to the router itself, the packets are filtered and
either processed locally or forwarded to the CE, depending upon the
instructions set-up by the CE. Otherwise, the FE determines the next
hop IP address by looking in its forwarding table. The FE also
determines the network interface it uses to send the packets.
Sometimes an FE may need to forward the packets to another FE before
packets can be forwarded out to the next hop. Right before packets
are forwarded out to the next hop, the FE decrements TTL by 1 and
processes any IP options that could not be processed before. The FE
performs IP fragmentation if necessary, determines the link layer
address (e.g., by ARP), and encapsulates the IP datagram (or each of
the fragments thereof) in an appropriate link layer frame and queues
it for output on the interface selected.
Other options mentioned in RFC 1812 [2] for IP forwarding may also be
implemented at FEs, for example, packet filtering.
FEs typically forward packets destined locally to CEs. FEs may also
forward exceptional packets (packets that FEs do not know how to
handle) to CEs. CEs are required to handle packets forwarded by FEs
for whatever reason. It might be necessary for ForCES to attach some
meta-data with the packets to indicate the reasons of forwarding from
FEs to CEs. Upon receiving packets with meta-data from FEs, CEs can
decide to either process the packets themselves, or pass the packets
to the upper layer protocols including routing and management
protocols. If CEs are to process the packets by themselves, CEs may
choose to discard the packets, or modify and re-send the packets.
CEs may also originate new packets and deliver them to FEs for
further forwarding.
Any state change during router operation must also be handled
correctly according to RFC 1812. For example, when an FE ceases
forwarding, the entire NE may continue forwarding packets, but it
needs to stop advertising routes that are affected by the failed FE.
5.5. Transport Layer
The Transport layer is typically implemented at CEs to support higher
layer application protocols like routing protocols. In practice,
this means that most CEs implement both the Transmission Control
Protocol (TCP) and the User Datagram Protocol (UDP).
Both CEs and FEs need to implement the ForCES Protocol. If some
layer-4 transport is used to support ForCES, then both CEs and FEs
need to implement the L4 transport and ForCES Protocols.
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5.6. Application Layer -- Routing Protocols
Interior and exterior routing protocols are implemented on CEs. The
routing packets originated by CEs are forwarded to FEs for delivery.
The results of such protocols (like forwarding table updates) are
communicated to FEs via ForCES.
For performance or scalability reasons, portions of the control plane
functions that need faster response may be moved from the CEs and
off-loaded onto the FEs. For example, in OSPF, the Hello protocol
packets are generated and processed periodically. When done at the
CEs, the inbound Hello packets have to traverse from the external
interfaces at the FEs to the CEs via the internal CE-FE channel.
Similarly, the outbound Hello packets have to go from the CEs to the
FEs and to the external interfaces. Frequent Hello updates place
heavy processing overhead on the CEs and can overwhelm the CE-FE
channel as well. Since typically there are far more FEs than CEs in
a router, the off-loaded Hello packets are processed in a much more
distributed and scalable fashion. By expressing such off-loaded
functions in the FE model, we can ensure interoperability. However,
the exact description of the off-loaded functionality corresponding
to the off-loaded functions expressed in the FE model are not part of
the model itself and will need to be worked out as a separate
specification.
5.7. Application Layer -- Network Management Protocol
RFC 1812 [2] also dictates that "Routers MUST be manageable by SNMP".
In general, for the post-association phase, most external management
tasks (including SNMP) should be done through interaction with the CE
in order to support the appearance of a single functional device.
Therefore, it is recommended that an SNMP agent be implemented by CEs
and that the SNMP messages received by FEs be redirected to their
CEs. AgentX framework defined in RFC 2741 ([6]) may be applied here
such that CEs act in the role of master agent to process SNMP
protocol messages while FEs act in the role of subagent to provide
access to the MIB objects residing on FEs. AgentX protocol messages
between the master agent (CE) and the subagent (FE) are encapsulated
and transported via ForCES, just like data packets from any other
application layer protocols.
6. Summary
This document defines an architectural framework for ForCES. It
identifies the relevant components for a ForCES network element,
including (one or more) FEs, (one or more) CEs, one optional FE
manager, and one optional CE manager. It also identifies the
interaction among these components and discusses all the major
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reference points. It is important to point out that, among all the
reference points, only the Fp interface between CEs and FEs is within
the scope of ForCES. ForCES alone may not be enough to support all
desirable NE configurations. However, we believe that ForCES over an
Fp interface is the most important element in realizing physical
separation and interoperability of CEs and FEs, and hence the first
interface that ought to be standardized. Simple and useful
configurations can still be implemented with only CE-FE interface
being standardized, e.g., single CE with full-meshed FEs.
7. Acknowledgements
Joel M. Halpern gave us many insightful comments and suggestions and
pointed out several major issues. T. Sridhar suggested that the
AgentX protocol could be used with SNMP to manage the ForCES network
elements. Susan Hares pointed out the issue of graceful restart with
ForCES. Russ Housley, Avri Doria, Jamal Hadi Salim, and many others
in the ForCES mailing list also provided valuable feedback.
8. Security Considerations
The NE administrator has the freedom to determine the exact security
configuration that is needed for the specific deployment. For
example, ForCES may be deployed between CEs and FEs connected to each
other inside a box over a backplane. In such a scenario, physical
security of the box ensures that most of the attacks, such as man-
in-the-middle, snooping, and impersonation, are not possible, and
hence the ForCES architecture may rely on the physical security of
the box to defend against these attacks and protocol mechanisms may
be turned off. However, it is also shown that denial of service
attacks via external interfaces as described below in Section 8.1.8
is still a potential threat, even for such an "all-in-one-box"
deployment scenario and hence the rate limiting mechanism is still
necessary. This is just one example to show that it is important to
assess the security needs of the ForCES-enabled network elements
under different deployment scenarios. It should be possible for the
administrator to configure the level of security needed for the
ForCES Protocol.
In general, the physical separation of two entities usually results
in a potentially insecure link between the two entities and hence
much stricter security measurements are required. For example, we
pointed out in Section 4.1 that authentication becomes necessary
between the CE manager and FE manager, between the CE and CE manager,
and between the FE and FE manager in some configurations. The
physical separation of the CE and FE also imposes serious security
requirements for the ForCES Protocol over the Fp interface. This
section first attempts to describe the security threats that may be
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introduced by the physical separation of the FEs and CEs, and then it
provides recommendations and guidelines for the secure operation and
management of the ForCES Protocol over the Fp interface based on
existing standard security solutions.
8.1. Analysis of Potential Threats Introduced by ForCES
This section provides the threat analysis for ForCES, with a focus on
the Fp interface. Each threat is described in detail with the
effects on the ForCES Protocol entities or/and the NE as a whole, and
the required functionalities that need to be in place to defend the
threat.
8.1.1. "Join" or "Remove" Message Flooding on CEs
Threats: A malicious node could send a stream of false "join NE" or
"remove from NE" requests on behalf of a non-existent or unauthorized
FE to legitimate CEs at a very rapid rate, and thereby creating
unnecessary state in the CEs.
Effects: If maintaining state for non-existent or unauthorized FEs, a
CE may become unavailable for other processing and hence suffer from
a denial of service (DoS) attack similar to the TCP SYN DoS. If
multiple CEs are used, the unnecessary state information may also be
conveyed to multiple CEs via the Fr interface (e.g., from the active
CE to the stand-by CE) and hence subject multiple CEs to a DoS
attack.
Requirement: A CE that receives a "join" or "remove" request should
not create any state information until it has authenticated the FE
endpoint.
8.1.2. Impersonation Attack
Threats: A malicious node can impersonate a CE or FE and send out
false messages.
Effects: The whole NE could be compromised.
Requirement: The CE or FE must authenticate the message as having
come from an FE or CE on the list of the authorized ForCES elements
(provided by the CE or FE Manager in the pre-association phase)
before accepting and processing it.
8.1.3. Replay Attack
Threat: A malicious node could replay the entire message previously
sent by an FE or CE entity to get around authentication.
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Effect: The NE could be compromised.
Requirement: A replay protection mechanism needs to be part of the
security solution to defend against this attack.
8.1.4. Attack during Fail Over
Threat: A malicious node may exploit the CE fail-over mechanism to
take over the control of NE. For example, suppose two CEs, say CE-A
and CE-B, are controlling several FEs. CE-A is active and CE-B is
stand-by. When CE-A fails, CE-B is taking over the active CE
position. The FEs already had a trusted relationship with CE-A, but
the FEs may not have the same trusted relationship established with
CE-B prior to the fail-over. A malicious node can take over as CE-B
if such a trusted relationship has not been established prior to or
during the fail-over.
Effect: The NE may be compromised after such insecure fail-over.
Requirement: The level of trust between the stand-by CE and the FEs
must be as strong as the one between the active CE and the FEs. The
security association between the FEs and the stand-by CE may be
established prior to fail-over. If not already in place, such
security association must be re-established before the stand-by CE
takes over.
8.1.5. Data Integrity
Threats: A malicious node may inject false messages to a legitimate
CE or FE.
Effect: An FE or CE receives the fabricated packet and performs an
incorrect or catastrophic operation.
Requirement: Protocol messages require integrity protection.
8.1.6. Data Confidentiality
Threat: When FE and CE are physically separated, a malicious node may
eavesdrop the messages in transit. Some of the messages are critical
to the functioning of the whole network, while others may contain
confidential business data. Leaking of such information may result
in compromise even beyond the immediate CE or FE.
Effect: Sensitive information might be exposed between the CE and FE.
Requirement: Data confidentiality between the FE and CE must be
available for sensitive information.
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8.1.7. Sharing security parameters
Threat: Consider a scenario where several FEs are communicating to
the same CE and sharing the same authentication keys for the Fp
interface. If any FE or CE is compromised, all other entities are
compromised.
Effect: The whole NE is compromised.
Recommendation: To avoid this side effect, it's better to configure
different security parameters for each FE-CE communication over the
Fp interface.
8.1.8. Denial of Service Attack via External Interface
Threat: When an FE receives a packet that is destined for its CE, the
FE forwards the packet over the Fp interface. A malicious node can
generate a huge message storm like routing protocol packets etc.
through the external Fi/f interface so that the FE has to process and
forward all packets to the CE through the Fp interface.
Effect: The CE encounters resource exhaustion and bandwidth
starvation on Fp interface due to an overwhelming number of packets
from FEs.
Requirement: Some sort of rate limiting mechanism MUST be in place at
both the FE and CE. The Rate Limiter SHOULD be configured at the FE
for each message type being received through the Fi/f interface.
8.2. Security Recommendations for ForCES
The requirements document [4] suggested that the ForCES Protocol
should support reliability over the Fp interface, but no particular
transport protocol is yet specified for ForCES. This framework
document does not intend to specify the particular transport either,
and so we only provide recommendations and guidelines based on the
existing standard security protocols [18] that can work with the
common transport candidates suitable for ForCES.
We review two existing security protocol solutions, namely IPsec (IP
Security) [15] and TLS (Transport Layer Security) [14]. TLS works
with reliable transports such as TCP or SCTP for unicast, while IPsec
can be used with any transport (UDP, TCP, SCTP) and supports both
unicast and multicast. Both TLS and IPsec can be used potentially to
satisfy all of the security requirements for the ForCES Protocol. In
addition, other approaches that satisfy the requirements can be used
as well, but are not documented here, including the use of L2
security mechanisms for a given L2 interconnect technology.
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When ForCES is deployed between CEs and FEs inside a box or a
physically secured room, authentication, confidentiality, and
integrity may be provided by the physical security of the box. Thus,
the security mechanisms may be turned off, depending on the
networking topology and its administration policy. However, it is
important to realize that even if the NE is in a single-box, the DoS
attacks as described in Section 8.1.8 can still be launched through
the Fi/f interfaces. Therefore, it is important to have the
corresponding counter-measurement in place, even for single-box
deployment.
8.2.1. Using TLS with ForCES
TLS [14] can be used if a reliable unicast transport such as TCP or
SCTP is used for ForCES over the Fp interface. The TLS handshake
protocol is used during the association establishment or re-
establishment phase to negotiate a TLS session between the CE and FE.
Once the session is in place, the TLS record protocol is used to
secure ForCES communication messages between the CE and FE.
A basic outline of how TLS can be used with ForCES is described
below. Steps 1) through 7) complete the security handshake as
illustrated in Figure 9, while step 8) is for all further
communication between the CE and FE, including the rest of the
messages after the security handshake shown in Figure 9 and the
steady-state communication shown in Figure 10.
1) During the Pre-association phase, all FEs are configured with the
CEs (including both the active CE and the standby CE).
2) The FE establishes a TLS connection with the CE (master) and
negotiates a cipher suite.
3) The FE (slave) gets the CE certificate, validates the signature,
checks the expiration date, and checks whether the certificate has
been revoked.
4) The CE (master) gets the FE certificate and performs the same
validation as the FE in step 3).
5) If any of the checks fail in step 3) or step 4), the endpoint must
generate an error message and abort.
6) After successful mutual authentication, a TLS session is
established between the CE and FE.
7) The FE sends a "join NE" message to the CE.
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8) The FE and CE use the TLS session for further communication.
Note that there are different ways for the CE and FE to validate a
received certificate. One way is to configure the FE Manager or CE
Manager or other central component as CA, so that the CE or FE can
query this pre-configured CA to validate that the certificate has not
been revoked. Another way is to have the CE and FE directly
configure a list of valid certificates in the pre-association phase.
In the case of fail-over, it is the responsibility of the active CE
and the standby CE to synchronize ForCES states, including the TLS
states to minimize the state re-establishment during fail-over. Care
must be taken to ensure that the standby CE is also authenticated in
the same way as the active CE, either before or during the fail-over.
8.2.2. Using IPsec with ForCES
IPsec [15] can be used with any transport protocol, such as UDP,
SCTP, and TCP, over the Fp interface for ForCES. When using IPsec,
we recommend using ESP in the transport mode for ForCES because
message confidentiality is required for ForCES.
IPsec can be used with both manual and automated SA and cryptographic
key management. But IPsec's replay protection mechanisms are not
available if manual key management is used. Hence, automatic key
management is recommended if replay protection is deemed important.
Otherwise, manual key management might be sufficient for some
deployment scenarios, especially when the number of CEs and FEs is
relatively small. It is recommended that the keys be changed
periodically, even for manual key management.
IPsec can support both unicast and multicast transport. At the time
this document was published, the MSEC working group was actively
working on standardizing protocols to provide multicast security
[17]. Multicast-based solutions relying on IPsec should specify how
to meet the security requirements in [4].
Unlike TLS, IPsec provides security services between the CE and FE at
IP level, so the security handshake, as illustrated in Figure 9
amounts to a "no-op" when manual key management is used. The
following outlines the steps taken for ForCES in such a case.
1) During the Pre-association phase, all the FEs are configured with
CEs (including the active CE and standby CE) and SA parameters
manually.
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2) The FE sends a "join NE" message to the CE. This message and all
others that follow are afforded security service according to the
manually configured IPsec SA parameters, but replay protection is
not available.
It is up to the administrator to decide whether to share the same key
across multiple FE-CE communication, but it is recommended that
different keys be used. Similarly, it is recommended that different
keys be used for inbound and outbound traffic.
If automatic key management is needed, IKE [16] can be used for that
purpose. Other automatic key distribution techniques, such as
Kerberos, may be used as well. The key exchange process constitutes
the security handshake as illustrated in Figure 9. The following
shows the steps involved in using IKE with IPsec for ForCES. Steps
1) to 6) constitute the security handshake in Figure 9.
1) During the Pre-association phase, all FEs are configured with the
CEs (including active CE and standby CE), IPsec policy etc.
2) The FE kicks off the IKE process and tries to establish an IPsec
SA with the CE (master). The FE (Slave) gets the CE certificate
as part of the IKE negotiation. The FE validates the signature,
checks the expiration date, and checks whether the certificate has
been revoked.
3) The CE (master) gets the FE certificate and performs the same
check as the FE in step 2).
4) If any of the checks fail in step 2) or step 3), the endpoint must
generate an error message and abort.
5) After successful mutual authentication, the IPsec session is
established between the CE and FE.
6) The FE sends a "join NE" message to the CE. No SADB entry is
created in FE yet.
7) The FE and CE use the IPsec session for further communication.
The FE Manager, CE Manager, or other central component can be used as
a CA for validating CE and FE certificates during the IKE process.
Alternatively, during the pre-association phase, the CE and FE can be
configured directly with the required information, such as
certificates or passwords etc., depending upon the type of
authentication that administrator wants to configure.
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In the case of fail-over, it is the responsibility of the active CE
and standby CE to synchronize ForCES states and IPsec states to
minimize the state re-establishment during fail-over. Alternatively,
the FE needs to establish a different IPsec SA during the startup
operation itself with each CE. This will minimize the periodic state
transfer across the IPsec layer though the Fr (CE-CE) Interface.
9. References
9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Baker, F., Ed., "Requirements for IP Version 4 Routers", RFC
1812, June 1995.
[3] Floyd, S., "Congestion Control Principles", BCP 41, RFC 2914,
September 2000.
[4] Khosravi, H. and Anderson, T., Eds., "Requirements for
Separation of IP Control and Forwarding", RFC 3654, November
2003.
9.2. Informative References
[5] Case, J., Mundy, R., Partain, D. and B. Stewart, "Introduction
and Applicability Statements for Internet Standard Management
Framework", RFC 3410, December 2002.
[6] Daniele, M., Wijnen, B., Ellison, M. and D. Francisco, "Agent
Extensibility (AgentX) Protocol Version 1", RFC 2741, January
2000.
[7] Chan, K., Seligson, J., Durham, D., Gai, S., McCloghrie, K.,
Herzog, S., Reichmeyer, F., Yavatkar, R. and A. Smith, "COPS
Usage for Policy Provisioning (COPS-PR)", RFC 3084, March 2001.
[8] Crouch, A. et al., "ForCES Applicability Statement", Work in
Progress.
[9] Anderson, T. and J. Buerkle, "Requirements for the Dynamic
Partitioning of Switching Elements", RFC 3532, May 2003.
[10] Leelanivas, M., Rekhter, Y. and R. Aggarwal, "Graceful Restart
Mechanism for Label Distribution Protocol", RFC 3478, February
2003.
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RFC 3746 ForCES Framework April 2004
[11] Moy, J., Pillay-Esnault, P. and A. Lindem, "Graceful OSPF
Restart", RFC 3623, November 2003.
[12] Sangli, S. et al., "Graceful Restart Mechanism for BGP", Work in
Progress.
[13] Shand, M. and L. Ginsberg, "Restart Signaling for IS-IS", Work
in Progress.
[14] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[15] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[16] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[17] Hardjono, T. and Weis, B. "The Multicast Group Security
Architecture", RFC 3740, March 2004.
[18] Bellovin, S., Schiller, J. and C. Kaufman, Eds., "Security
Mechanisms for the Internet", RFC 3631, December 2003.
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10. Authors' Addresses
L. Lily Yang
Intel Corp., MS JF3-206,
2111 NE 25th Avenue
Hillsboro, OR 97124, USA
Phone: +1 503 264 8813
EMail: lily.l.yang@intel.com
Ram Dantu
Department of Computer Science,
University of North Texas,
Denton, TX 76203, USA
Phone: +1 940 565 2822
EMail: rdantu@unt.edu
Todd A. Anderson
Intel Corp.
2111 NE 25th Avenue
Hillsboro, OR 97124, USA
Phone: +1 503 712 1760
EMail: todd.a.anderson@intel.com
Ram Gopal
Nokia Research Center
5, Wayside Road,
Burlington, MA 01803, USA
Phone: +1 781 993 3685
EMail: ram.gopal@nokia.com
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RFC 3746 ForCES Framework April 2004
11. Full Copyright Statement
Copyright © The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
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RFC TOTAL SIZE: 98660 bytes
PUBLICATION DATE: Monday, April 26th, 2004
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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